<!DOCTYPE html>
<html>
<head>
<style type="text/css">
#a {
 background-color: green;
 margin-bottom: 20px;
}
#a div {
 height: 20px;
 margin-bottom: 10px;
}
#b {
 height: 20px;
 background-color: green;
 margin-top: 40px;
}
</style>
</head>
<body>
<div id="a">
 <div></div>
</div>
<div id="b">
</div>
</body>
</html>
